Burner

ABSTRACT

A burner capable of producing a low NO x  emission combustion in the whole operating range, maintaining the flame at all times anchored to the diffuser body of the burner and without the use of systems, for example valves, with which to switch the introduction position of the reactants according to the temperature of the furnace. Such a burner is more flexible and cost-effective, both from a system point of view and from a control point of view, with respect to traditional burners.

FIELD OF THE INVENTION

The present invention relates to a burner, particularly to a low NO_(x) emission burner.

STATE OF THE ART

Diffusion flame burners are normally conceived to inject air and gas through nozzles of various shape and type and are provided with a flame guiding cone whose function is to anchor the combustion to the burner itself, avoiding detachment of the flame.

Burners with different profiles and flame shapes may be obtained according to the type of air and gas diffusers and the shape of the flame guiding cone.

In order to obtain burners characterised by low NO_(x) emissions there are various technologies whose principles essentially consist in achieving the combustion so as to reduce the flame temperatures which give rise to the formation of thermal NO_(x).

The most used techniques are essentially two:

a combustion in several “stages”,

or creating the combustion reaction after that the reactants, air and gas, have been partially mixed and “diluted” with partial combustion products, typically the fumes existing in the chamber onto which the burner opens.

Both techniques, in order to obtain effective results in terms of NO_(x) reduction, require a burner configuration which contemplates the double input position of either air, or gas, or both. The reason consists in the fact that, when cold, both the “stage” combustion and the “diluted” combustion do not allow to maintain the combustion reaction stable. For this reason, the burner is constructed so that there are provided several positions or input points where to inject the reactants. When cold, when the burner is ignited and the heating of the chamber starts, the combustion occurs traditionally (diffusion flame) and with a high NO_(x) emission level; when the temperature exceeds the autoignition value of the gas, the introduction point of either the air, or the gas, or both the reactants is switched by means of valves, thus passing to low NO_(x) emission combustion. For safety reasons, the switching occurs at temperatures higher than those of autoignition of the gas because normally, after conversion to the low NO_(x) emission condition, flame detachment occurs.

Burners referred to as “flameless”, characterised by a completely diffused combustion not anchored to the mixer-diffuser of the burner, also work with the same principle. These burners moreover differ for the fact that, in low NO_(x) mode, the presence of the flame cannot be detected by the traditional automatic UV cell detection system or by ionisation electrode.

The known burners, therefore, have the disadvantage of producing a high level of NO_(x) emissions when cold, while, when hot, in order to obtain low emissions, they must be provided with reactant introduction point switching systems which make the burner complex from a constructive and functional point of view, and consequently more expensive. An example of a known burner is described in the document U.S. Pat. No. 6,206,686. Disadvantageously, such a burner provides two separate combustible gas lines and the flow rate distribution of such a gas must be balanced between the first and the second stage during the heating-up stage of the furnace. This means that NO_(x) emissions are reduced only when the temperature exceeds the autoignition value and that an active control system which adjusts the gas flow rates on the two lines according to temperature must be contemplated. Furthermore, this burner requires an ignition pilot with corresponding dedicated air and gas plant.

The need to construct a burner capable of overcoming the aforesaid drawbacks is therefore felt.

SUMMARY OF THE INVENTION

It is the primary object of the present invention to construct a burner capable of producing in every operative condition a combustion with low NO_(x) emissions also during the cold stage, maintaining the flame at all times anchored to the body of the burner and without the use of systems, for example valves, with which to switch the reactant introduction position.

Another object of the invention is to be able to continuously survey the flame, at least the flame that develops in the first stage of combustion, to increase security.

A further object is to construct a more flexible and inexpensive burner, both from a plant point of view and from a control point of view.

The present invention, therefore, proposes to reach the objects discussed above by constructing a burner that, according to claim 1, comprises:

a comburent gas inlet opening,

a combustible gas inlet opening,

a first external tubular body provided at a respective first end with one or more respective ports for the outlet of part of comburent gas, and connected to the comburent gas inlet opening;

a second tubular body arranged within the first tubular body and communicating with the latter, provided at a respective first end with one or more respective ports for the outlet of another part of comburent gas,

a third tubular body arranged within the second tubular body, provided at a respective first end with one or more respective ports for the outlet of part of combustible gas, and connected to the combustible gas inlet opening;

a fourth tubular body arranged within the third tubular body and communicating with the latter, provided at a respective first end with one or more respective ports for the outlet of another part of combustible gas,

wherein the respective first ends of the first, second, third, fourth tubular bodies are arranged in proximity of a same first end of the burner so that two combustion zones of comburent gas with combustible gas are defined at the same first end of the burner.

A further aspect of the invention provides a combustion process, achievable by means of the aforesaid burner, according to claim 11.

Advantageously, the burner, object of the present invention, is provided with a single combustible gas inlet pipe or opening, connected to the third tubular body, and may be ignited and run stably also when cold without the need to actively adjust the combustible gas flow rates between first and second stages of the combustion, maintaining the NO_(x) emissions very low at all times.

A further advantage is represented by the fact that said burner is provided with a single inlet pipe or opening of the comburent gas, generally air, connected to the first tubular body, which opportunely splits the flow rate into two introduction points: the first point in the innermost combustion zone, the second point in the most advanced part of the burner so as to produce a combustion recirculation and dilution effect, induced by the high speed jets.

Advantageously, the communication holes between the first and the second tubular bodies are sized in function of the distribution of a predetermined flow rate of comburent gas; analogously the communication holes between the third and the fourth tubular bodies are sized in function of the distribution of a predetermined flow rate of combustible gas.

Advantageously, the burner according to the invention is capable of operating, straight from the ignition stage in which the temperatures are low, maintaining at all times very low NO_(x) values without modifying the position or the introduction mode of the reactants.

Furthermore, the burner is equipped with a direct spark plug ignition system and with an electrode flame detection system which constantly detects the flame in all operating conditions to the advantage of safety. An ignition pilot with corresponding dedicated air and gas plant is consequently not needed.

Such burner is, therefore, particularly suitable to be used in thermal treatment furnaces or reheating furnaces in which the temperatures oscillate about the autoignition limit of the gas; indeed, traditional burners in this range of temperatures are subjected to continuous valve switchings.

The dependent claims describe preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will be more apparent in the light of the detailed description of a preferred, but not exclusive, embodiment of a burner illustrated by way of non-limitative example, with the help of the accompanying drawings, in which:

FIG. 1 shows a longitudinal section, along a first plane, of a burner according to the invention;

FIG. 2 shows a longitudinal section of the burner along a second plane;

FIG. 3 shows an axonometric view of the burner of FIGS. 1 e 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

With reference to the FIGS. 1 to 6 it is shown a preferred embodiment of the burner according to the invention comprising a metallic body, indicated as a whole with reference numeral 1, provided with a plurality of tubes, an inlet flange 6 of the comburent gas, preferably air, and an inlet flange 7 of the combustible gas. Comburent gas inlet flange 6 opportunely communicates with a first external tube 2 of metallic body 1 and with a second tube 3, arranged within the first tube 2, at one end thereof. The communication is such that the air or other comburent gas is split into predetermined proportions in the first and second tube. Preferably, such proportions are approximately 50-70% in tube 2 and approximately 30-50% in tube 3.

Advantageously the second tube 3, provided for the passage of comburent gas, is provided, on the surface at one end, with a series of radial holes 9′, having axes arranged substantially radially with respect to a section of the tube, which allow the communication with first tube 2, also this provided for the passage of comburent gas, and is provided, at its other end, with one or more respective ports 9 for the outlet of a first part of the comburent gas in a first combustion zone. More particularly these ports 9 are defined by a plurality of openings, having substantially axial direction, and are made on a diffuser body 13 having substantially hollow cylindrical shape and arranged transversally to the interior of the second tube 3. The diffuser body 13 is staggered in respect of the end section of tube 3.

Preferably, the radial holes 9′ are arranged along at least one circumference of a transversal section of the second tube 3, preferably in correspondence of the inlet flange 6 for a better flow rate distribution.

Similarly, first tube 2 is provided at its other end with one or more respective ports 8 for the outlet of a second part of the air in a second combustion zone. More particularly said ports 8 are defined by a plurality of openings having substantially axial direction and arranged along a circular crown at the end of the tube 2. In a preferred embodiment, there are four ports 8, made on an element of refractory material, solidarily applied to the end of the first tube 2 and having a substantially semi-elliptical shape. Said element of refractory material is resistant to thermal shock and is, for example, made of silicon carbide or alumina-based material.

Therefore, with regard to the tubes for the comburent gas, the ports 9 of the second tube 3 are arranged in more internal position with respect to ports 8 of the first tube 2.

Combustible gas inlet flange 7 communicates, instead, with a third tube 4, arranged within second tube 3, and with a fourth tube 5, arranged within said third tube 4, at an end thereof. The communication is such to split a minimum of 5÷35% of the combustible gas flow rate in tube 4, for maintaining the flame, and the remaining 65÷95% of the flow rate in tube 5.

The third tube 4 ends at its second extremity in the cavity of the diffuser body 13, whereas the fourth tube 5 ends in a more advanced position near the end section of the tubes 2 and 3. In this manner the following outflow ports of the fuel gas are defined:

port 10, corresponding to the annulus placed between the diffuser body 13 and the fourth tube 5, though which a first part of the fuel gas outflows in the first combustion zone; alternatively port 10 can be constituted by a plurality of smaller ports;

port 11, corresponding to the end section of the fourth tube 5 through which outflows a second part of the fuel gas in the second combustion zone; alternatively port 11 can be constituted by a plurality of smaller ports.

Consequently the port 10 of the third tube 4 is advantageously placed in an inner position with respect to the port 11 of the fourth tube 5.

Advantageously, with regard to the tubes for the combustible gas, the fourth tube 5 is provided, on the surface at one end, with a series of radial holes 11′ which allow the communication with third tube 4. Preferably, radial holes 11′ are arranged along at least one circumference of a transversal section of the fourth tube 5, preferably in correspondence of the inlet flange 7 for a better flow rate distribution.

A preferred embodiment of the burner provides that the four tubes 2, 3, 4, 5 are cylindrical and coaxial. Third and fourth tubes 4, 5 may constitute, for example, a double concentric gas lance.

Furthermore, in fourth tube 5, an inlet opening 12 of inert gas, such as nitrogen, for cooling the burner, particularly the double lance, may be advantageously provided.

A further advantage is due to the fact that the burner of the invention is provided with two separate electrodes 16 for ignition and flame detection. The two electrodes 16 are visible in the section shown in FIG. 2. These are arranged so as to cross second tube 3 in the longitudinal direction and to be engaged in the diffuser body 13. At the end of the burner, in which there are provided the comburent gas and combustible gas outlet ports of the corresponding tubes, a combustion area is defined.

Due to the particular arrangement of outlet ports 8, 9, 10, 11, combustion advantageously occurs in two stages defining as set forth above two combustion zones.

The first combustion stage occurs with combustion ratio defined according to the splitting of comburent gas flow rates supplied by ports 9 and combustible gas flow rates supplied by port 10.

This condition is very favourable to restrain NO_(x) emissions because only a small part of the combustible gas burns and the flame temperatures within the first combustion zone are maintained low because the generated heat is mostly absorbed by the reactants which do not take part in the first stage combustion. The first combustion stage products encounter the remaining part of comburent gas and combustible gas exiting from ports 8 and 11, respectively.

Advantageously, the flow of comburent gas passing at high speed through the ports 8 favours, by means of the Venturi effect, an aspiration and the recirculation of part of the combustion products coming from the first stage and part of the combustion products which already are, for example, within the chamber of a thermal treatment furnace, said combustion products being in this manner mixed with the combustion itself.

The comburent air outflows from the ports 8 at a speed variable in the range 100÷140 m/s . The combustible gas jet which exits from port 11 at high speeds, variable in the range 40÷70 m/s, contributes to increasing the suction effect of the combustion products present in the furnace chamber, with a two-fold effect:

an increase of the temperature of second stage reactant gas before combustion;

and a simultaneous reduction of the percentage of oxygen in the gaseous fluid volume which will take part in and complete the combustion of the gases coming from the first stage, of the air and of the gas introduced in the second stage of the burner in points 8 and 11 and finally of the combusted gases already present in the furnace chamber.

This combination of effects contributes to “diluting” the flame with combustion products, by now inert and relatively colder with respect to the flame temperature, in wider volumes reducing the flame temperature peaks which cause the formation of NO_(x).

The minor part of the combustion which occurs at the first stage remains stable and always anchored to the diffuser body 13, which continues to receive part of the comburent air and the combustible gas and which maintains the combustion ignited in any condition of operation and therefore allows continuous flame detection, for instance by means of a ionisation electrode, both when hot and when cold.

Combustion continues and is completed downstream of the second stage where the remaining part of the combustible gas is injected through port 11 of fourth tube 5 and where the remaining part of the comburent air is injected through ports 8 of first tube 2 at high speed so as to promote the recirculation of fumes existing in the furnace chamber and to dilute the global combustion reaction.

Therefore, the burner according to the invention is capable of producing low NO_(x) emissions in all combustion conditions, also when the furnace chamber is at lower temperatures than those of autoignition of the combustible gas, exploiting both the principles of stage combustion and of diluted combustion with inert products present in the furnace chamber. 

1. A burner comprising: a comburent gas inlet opening, a combustible gas inlet opening, a first external tubular body provided at a respective first end with one or more respective ports for the outlet of part of comburent gas, and connected to the comburent gas inlet opening; a second tubular body arranged within the first tubular body and communicating with the latter, provided at a respective first end with one or more respective ports for the outlet of another part of comburent gas, a third tubular body arranged within the second tubular body, provided at a respective first end with one or more respective ports for the outlet of part of combustible gas, and connected to the combustible gas inlet opening; a fourth tubular body arranged within the third tubular body and communicating with the latter, provided at a respective first end with one or more respective ports for the outlet of another part of combustible gas, wherein the respective first ends of the first, second, third, fourth tubular bodies are arranged in proximity of a same first end of the burner so that two combustion zones of comburent gas with combustible gas are defined at the same first end of the burner.
 2. A burner according to claim 1, wherein the one or more respective ports of the second tubular body are arranged in more internal position with respect to the one or more respective ports of the first tubular body.
 3. A burner according to claim 2, wherein the one or more respective ports of the third tubular body are arranged in more internal position with respect to the one or more respective ports of the fourth tubular body.
 4. A burner according to claim 3, wherein the second tubular body is provided with radial holes communicating with the first body.
 5. A burner according to claim 4, wherein said radial holes are arranged at a second end of the second body, preferably at the comburent gas inlet opening.
 6. A burner according to claim 4, wherein the fourth tubular body is provided with radial holes communicating with the third body.
 7. A burner according to claim 6, wherein said radial holes are arranged at a second end of the fourth body, preferably at the combustible gas inlet opening.
 8. A burner according to claim 1, wherein the first, second, third and fourth tubular bodies are cylindrical and coaxial.
 9. A burner according to claim 1, wherein in the fourth tubular body there is provided an inlet opening of inert gas for a cooling of the burner.
 10. A burner according to claim 1, wherein there are provided two electrodes respectively to ignite and to detect a flame, passing within the second body and accommodated within the diffuser body.
 11. A combustion process in a burner according to claim 1, comprising the following stages: inlet of comburent gas, through an opening, in a first and a second tubular body in first predetermined proportions, inlet of combustible gas, through an opening, in a third and a fourth tubular body in second predetermined proportions, a first combustion stage by means of injection at a first combustion zone of part of the combustible gas through the one or more ports of the third tubular body and of part of the comburent gas through the one or more ports (9) of the second tubular body, a second combustion stage by means of injection at a second combustion zone of another part of the combustible gas through the one or more ports of the fourth tubular body and of another part of the comburent gas through the one or more ports of the first tubular body, wherein the first combustion stage occurs with a stoichiometric ratio in high excess of combustible gas so as to reduce the flame temperature and the formation of NO_(x) and to allow the detection of the flame both when hot and when cold, and wherein the second combustion stage comprises a recirculation of combustion products of said first stage so as to dilute the combustion again reducing the flame temperature and the formation of NO_(x).
 12. A combustion process according to claim 11, wherein the first predetermined proportions are equal to approximately 50-70% in the first body and to approximately 30-50% in the second body.
 13. A combustion process according to claim 11, wherein the second predetermined proportions are equal to approximately 5-35% in the third body and to approximately 65-95% in the fourth body.
 14. A combustion process according to claim 11, wherein the outlet speed of the comburent gas from the ports of the first body and the outlet speed of the combustible gas from the one or more ports of the fourth tubular body is such to cause said recirculation of combustion products of the first stage. 